I would look at some of the remakes since the PSP and DS have the definitive versions of so many of them. The Dragon Quest Zenithian trilogy is fantastic, with DQV being the best in my opinion (and one of the best RPGS I ever played). If you're into emulation on your PSP, the fanslation is very playable and has aged pretty well. Pokemon Heart Gold is great as well and I actually enjoyed it more than its sequels which were originally designed and released on the DS.

For newer games, Valkyria Chronicles II is one of my favorites on PSP. You don't have to play the first to enjoy it and the gameplay is a fresh evolution of SRPG genre. It did a great job in bringing the series to the portable format, with plenty of bite sized missions and story sequences for quick sessions. But you can get sucked in for a few hours at a time too.